In recent years, there has been a highly increasing demand for electrochemical devices for various portable devices and hybrid electric vehicles. A concentrated sulfuric acid aqueous solution, a concentrated alkaline aqueous solution and the like have been used as electrolytes of electrochemical devices such as secondary batteries and capacitors. Further, non-aqueous solvents with lithium salt dissolved therein have been used as electrolytes of lithium ion secondary batteries. Many of the electrolytes cause corrosion of materials or damage to the human body because of liquid leakage thereof.
It is thus essential to prevent the liquid leakage of the electrolyte in the electrochemical device; however the electrolyte has a tendency to diffuse on a metal part. The phenomenon of diffusion of the electrolyte on the metal part is called creeping. M. N. Hull et al. gave the following causes of creeping of the electrochemical device using an alkaline electrolyte, in “J. Electrochem. Soc., 124, 3, 332 (1977)”: (i) a decrease in surface tension of the electrolyte due to an electrocapillary action, (ii) an increase in alkaline concentration caused by reduction of oxygen on the interface of three phases formed by the electrolyte, oxygen and a negative electrode, and (iii) the transportation of the electrolyte driven by the increasing alkaline concentration.
When an adhesive is applied to a sealing part of the electrochemical device, with the affinity between the metal part and the electrolyte higher than the affinity between the adhesive and the metal part, the adhesive tends to be separated from the metal part by the action of the electrolyte. It has thus been difficult to prevent the liquid leakage of the electrolyte by the use of an adhesive.
Japanese Laid-Open Patent Publication No. 2000-58031 proposes the use of a hermetic seal. The hermetic seal has been in wide use for sealing electronic components as having high insulating properties and air-tightness. Since a thermal expansion coefficient of glass needs to be coordinated with a thermal expansion coefficient of a metal, however, it has been difficult to design the hermetic seal, limiting metals to expensive ones that can actually be used.
In a sealing part of a conventional electrochemical device, for example, a metal part having a surface layer made of butyl rubber and a gasket such as an O-ring in close contact with the surface layer have been used. In a capacitor including an electrolyte aqueous solution, a cylindrical rubber plug, having a through hole for passage of a current collecting terminal, has been used as a component for sealing an opening of a metal case.
However, in the aforesaid methods of physically blocking the electrolyte, there are required a step of compressing the gasket with the metal part, a step of caulking the rubber plug with the end of the metal case, and the like. This may hence raise problems of an increased production cost and of limited design of electrochemical devices. In the case of the capacitor, since a problem may arise that the electrolyte evaporates from the gap between the rubber plug and the current collecting terminal to cause an increase in internal resistance, reliable and inexpensive alternative techniques have been required.
Recently proposed has been an electrochemical device produced by stacking in series bipolar electrode plates each comprising a positive electrode, a negative electrode and a bipolar current collector carrying the positive electrode on one face thereof and the negative electrode on the other face thereof. In the electrochemical device comprising the bipolar electrode plate, it is of importance to prevent a short circuit between the electrode plates through the medium of the electrolyte. For this reason, Japanese Patent Publication No. 2623311, Japanese Laid-Open Patent Publication No. 11-204136 and Japanese Patent Publication No. 2993069 propose arranging an insulating material or a liquid impermeable material on the periphery of the bipolar current collector.
However, in the electrochemical device comprising the bipolar electrode plate, it is difficult to control the creeping of the electrolyte. Further, as long as a liquid electrolyte is in use, it is also difficult to prevent the transfer of the electrolyte between the electrode plates caused by fall or vibration of the electrochemical device. When the electrolyte causes occurrence of the short circuit between the electrode plates, problems of variation in charged state between the electrode plates, an increase in self-discharge, and the like, may arise. Meanwhile, an extreme reduction in amount of the electrolyte in order to prevent the transfer thereof results in shorter longevity of the electrochemical device. Accordingly, effective methods for preventing the short circuit between the stacked electrode plates caused by the electrolyte have been desired.
In recent times, sealed alkaline storage batteries have been used for power sources of electric power tools and hybrid electric vehicles. Because this usage necessitates a high-voltage power source, plural batteries connected in series are used.
However, a typical alkaline storage battery has: a case to accommodate, together with an electrolyte, an electrode plate assembly comprising a positive electrode plate, a negative electrode plate and a separator; and a sealing plate to seal the opening of the case. The cases and the sealing plates in number equivalent to the number of the batteries to be connected in series are thus needed, which is uneconomical. It is therefore considered that an effective manner is parting the inside of a closed-bottom resin case into plural spaces, inserting an element for power generation into each of the isolated spaces, and connecting in series the elements for power generation with a lead (lead wire), as in the case of a lead storage battery.
Compared to sulfuric acid to be used in the lead storage battery, however, an alkaline electrolyte to be used in the alkaline storage battery is more apt to creep on the surface of the metal part such as the lead. For this reason, there may be cases in which the elements for power generation connected in series are short-circuited through the medium of the electrolyte, leading to an increase in self-discharge or to variation in charged state between the elements for power generation. It is thus difficult to apply the structure of the lead storage battery, as it is, to that of the sealed alkaline storage battery.